This invention relates to printers, with which a dotted print of the data displayed on, for instance, a CRT display according to a video signal is obtained on a printing sheet.
A discharge printer using electric discharge such as spark discharge, for instance, has been used for printing given characters and figures as a dotted print on a printing sheet according to a video signal. In this discharge printer, the input video signal is sampled according to a sampling signal obtained from a phase locked loop (PLL), and the sampled video signal is used to cause spark discharge so as to decompose an aluminum film of an aluminum-deposited printing sheet, thus forming a dot. A dotted print of given characters or figures is obtained as corresponding combinations of dots thus formed.
With this kind of printer, however, the video signal portions corresponding to dots randomly appear and are not synchronized to a sampling signal which has a predetermined frequency. Therefore, it is likely that the rising or falling end of a portion corresponding to a dot is sampled and that the level of the sampled output becomes higher or lower than a threshold level which is a reference level of printing. Particularly, it has hitherto been the case that a black dot is printed even if the sampled output level is higher than the threshold level only slightly. In this case, the form of the printed characters or figures is inaccurate and is prone to misreading. Shown in (a), (b) and (c) in FIG. 1 are printed characters "A", "B" and "1" obtained with a conventional discharge printer. Here, labeled M are misprinted dots.
As is shown, with the prior art printer the misprinted dots have the same tone as the regular dots constituting the printed characters "A", "B" and "1", and therefore the printed characters have inaccurate contours and are sometimes liable to be read as different characters. This has been known as a grave drawback of the prior art discharge printer of this kind.